System and method for objective self-diagnosis of measurement device calibration condition

ABSTRACT

A measurement system uses a plurality of transducers that may differ from each other in at least one respect, such as having different operating principles or being made by different manufacturers. Respective measurement values obtained from the transducers are applied to a processor which provides a measured value based on the measurement values from a plurality of the transducers. The processor also provides information about the calibration drift of each of the transducers based upon a comparison between the measurement value obtained from the transducer to a value obtained from a combination of respective measurement values obtained from a plurality of the transducers. The calibration drift information provides an objective evaluation about the calibration condition of each of the transducers. When a transducer is determined to be outside of its calibration tolerance, a calibration needed alert occurs.

TECHNICAL FIELD

This application relates to measurement systems and methods, and, moreparticularly, to a measurement system and method that allows calibrationperiods to be extended with very low risk of overextending withoutimpairing measurement accuracy.

BACKGROUND OF THE INVENTION

Every measurement device drifts or changes with time. As a result ofthis drift, every measurement device must be recalibrated regularly toassure that measurements made by the device remain within a tolerancethat is normally defined by specifications of the measurement device.Failure to calibrate the measurement device before it is out oftolerance may have negative consequences, including recalling orquestioning all measurements made by the device since the lastcalibration. A failure to calibrate can also result in erroneousmeasurements in the field, which can be disastrous in terms of quality,cost, safety, etc. It is therefore highly desirable to calibrate atsufficiently short recalibration intervals.

Although a short recalibration interval is desirable to ensure accuratemeasurements, a recalibration interval that is shorter than necessaryincreases calibration frequency thus increasing direct and indirectcalibration costs, such as instrument downtime, shipping and associatedrisks. Currently, the recalibration interval is usually based on pastexperience with similar devices, and it is selected based on the largestanticipated calibration drift of the measurement device.

The difficulty in selecting an adequately short recalibration intervalis exacerbated by the fact that calibration drift occurs in differentmeasurement devices at different rates. Therefore, the recalibrationinterval of a measurement device is usually statistically determined onthe basis of the calibration drift history of a population of similardevices. An interval is then chosen that provides an acceptablelikelihood of in-tolerance conditions at calibration. Given the veryhigh cost of out of tolerance conditions at recalibration, the intervalmust be conservative, and it therefore causes a majority of measurementdevices to be calibrated earlier than necessary thereby unnecessarilyincreasing costs. In addition, the predictive statistical method has nochance of identifying outliers whose behavior deviates significantlyfrom predicted behavior.

There is therefore a need for a system and method that provides analternative to predictive recalibration interval selection so that alonger recalibration interval can be used without risking out oftolerance conditions at recalibration.

SUMMARY

A measurement system and method makes a plurality of measurements atleast some of which are measured independently of the othermeasurements. The system then derives a measured value from acombination of values obtained from at least some of the plurality ofrespective measurements. The plurality of measurements obtained by thesystem and method are also used to provide information about calibrationdrift of measurement devices used to make the measurements, such as anindication of whether the accuracy of at least some of the plurality ofmeasurements is less than a predetermined accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a pressure measurement system according toone embodiment of the invention.

FIG. 2 is a side isometric view of a pressure measurement deviceaccording to one embodiment of the invention.

FIG. 3 is a bottom isometric view of the pressure measurement device ofFIG. 2.

FIG. 4 is a block diagram of an electrical measurement system accordingto one embodiment of the invention.

DETAILED DESCRIPTION

A measurement system 10 according to one embodiment of the invention isshown in FIG. 1. The system 10 is used for measuring pressure, but theprinciple of operation could be used for a system measuring any otherkind of physical variable, such as temperature, or a characteristic ofan electrical signal, such a voltage or frequency. The system 10includes three pressure transducers 12, 14, 16 having respective inletports that are connected to a common pressure conduit 18. The pressuretransducers 12, 14, 16 are powered by respective current sources 22, 24,26 that are powered by a common power supply 28. Each of the currentsources 22, 24, 26 includes a reference voltage source 30 a,b,c thatprovides a regulated voltage, and a constant current source 32 a,b,cthat converts a reference voltage from the voltage source 30 a,b,c to acorresponding current. However, different types of power supplies may beused depending upon the nature of measurement devices used in a system.Also, although the system 10 uses three pressure transducers 12, 14, 16and associated components, only two pressure transducers or more thanthree pressure transducers could be used. However, three pressuretransducers 12, 14, 16 is the minimum number of transducers that make itpossible to identify a pressure transducer providing measurements thatdiffer markedly from the others.

Each of the pressure transducers 12, 14, 16 provides an analog outputvoltage that corresponds to the pressure in the conduit 18 measured bythe transducers 12, 14, 16. The output voltages are applied torespective analog-to-digital (“A/D”) converters 42, 44, 46, which outputrespective digital output signals indicative of the pressures measuredby the transducers 12, 14, 16, respectively. The digital output signalsfrom the A/D converters 42, 44, 46 are applied to a digital processor48, which may be, for example, a computer system running an applicationprogram performing a suitable analysis algorithm, as discussed ingreater detail below. The digital processor 48 determines and providesan indication of not only a measured pressure, but also informationrelating to the calibration drifts of the pressure transducers 12, 14,16.

A variety of algorithms can be used to determine the measured pressureand the calibration drift. A value for the measured pressure can beobtained, for example, from an average or a weighted average of thepressure measurements obtained from each of the pressure transducers 12,14, 16. In some embodiments, a measured pressure value is obtained fromthe measurements of only two of the pressure transducers 12, 14, 16, andthe value measured by one of the transducers 12, 14, 16 is ignoredbecause, for example, the pressure value differs excessively from thepressure measured by the other two pressure transducers 12, 14, 16. Thecalibration drift can be obtained by a variety of means. For example,the value of the pressure measured by each of the pressure transducers12, 14, 16 can be compared to the average or a weighted average of themeasured pressure values. If the value of the pressure measured by anyof the transducers 12, 14, 16 differs from the average by more than aspecific value, a calibration alert indicating that the transducers 12,14, 16 need to be calibrated can be provided. Other means of detectingthat calibration drift has reached a specific value will be apparent toone skilled in the art.

The transducers 12, 14, 16 may be identical to each other, or two ormore may be different from each other in a variety of respects. Forexample the pressure transducers 12, 14, 16 may have differentprinciples of operation, such as the transducer 12 having a diaphragminstrumented with resistance strain gauges, the transducer 14 having adiaphragm instrumented with resonant beam force sensor, and thetransducer 16 having a spring compressed or tensioned responsive topressure changes and instrumented with a position sensor that senses theposition of a movable end of the spring. As another example, thepressure transducers 12, 14, 16 may all use a common principle ofoperation, but they may be made by different manufacturers.Alternatively, they may all use a common principle of operation and bemade by the same manufacturer, but be from different manufacturing lots.Other variations between the pressure transducers 12, 14, 16 are alsopossible. Using pressure transducers 12, 14, 16 that differ from eachother in some respects tends to lessen the probability that theircalibration drifts will match each other.

An embodiment of a pressure sensor 50 is shown in FIGS. 2 and 3. Thepressure sensor 50 includes three pressure transducers 52, 54, 56 thatmay be identical to each other or they may be different, as explainedabove. Each of the transducers 52, 54, 56 includes electrical terminals58 through which the transducer 52, 54, 56 is powered and from which asignal indicative of the measured pressure can be obtained. Thetransducers may be symmetrically positioned in three 90 degree axesabout a pressure port 60 (FIG. 3) that extends from a spherical housing62. Also, a temperature probe 64 (FIG. 2) may be positioned equidistantfrom all of the transducers 52, 54, 56. Symmetrically positioning thetransducers 52, 54, 56 on a sphere equidistant from the test pressureport ensures that they are all exposed to the pressure in the samemanner even if the pressure changes so that valid comparisons can bemade between the pressures measured by the transducers. Orienting thetransducers in three 90 degree axes provides independence of the overallsensor from orientation and gravity effects. In use, the pressure port60 may face in any direction.

A measurement system 70 according to another embodiment of the inventionis shown in FIG. 4. The system 70 measures the voltage on a wire 72using three separate voltage measuring devices 74, 76, 78. Each of thevoltage measuring devices 74, 76, 78 provides a respective digitaloutput to a voltage processor 80, which uses a suitable algorithm asexplained above to provide a measured voltage value and informationrelating to calibration drift, such as a calibration needed alarm.Again, the voltage measuring devices 74, 76, 78 may be identical to eachother, or they may differ from each other in a variety of respects aspreviously explained. The voltage measuring devices 74, 76, 78 may beconnected to the wire 72 at the same location so that they are exposedto the same measurement environment. For example, if the voltagemeasuring devices 74, 76, 78 were connected to the wire 72 at differentlocations, a voltage drop might exist along the length of the wire, or avoltage could be capacitively coupled to one location on the wire 72 toa greater or less extend than to another location.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A method of measuring a parameter, comprising: making a plurality ofmeasurements at least some of which are measured independently of theother measurements; deriving a measured value from at least some of theplurality of measurements; and using the plurality of measurements todetermine whether the accuracy of at least some of the plurality ofmeasurements is less than a predetermined accuracy.
 2. The method ofclaim 1 wherein the act of making a plurality of measurements comprisesmaking a plurality of measurements using a respective plurality ofmeasurement devices having different principles of operation.
 3. Themethod of claim 1 wherein the act of making a plurality of measurementscomprises making a plurality measurements using a respective pluralityof measurement devices manufactured by different manufacturers.
 4. Themethod of claim 1 wherein the act of deriving a measured value from atleast some of the plurality of measurements comprises: discarding atleast one of the measurements based on the value obtained from the atleast one measurement compared to a combination of the values obtainedfrom at least some of the other measurements; and using the remainingmeasurement to provide the measured value.
 5. The method of claim 1wherein the act of deriving a measured value from at least some of theplurality of measurements comprises averaging the plurality ofmeasurements.
 6. The method of claim 1 wherein the act of using theplurality of measurements to determine whether the accuracy of at leastsome of the plurality of measurements is less than a predeterminedaccuracy comprises comparing the respective values obtained from each ofthe plurality of measurements to a value determined from a combinationof values obtained from the plurality of measurements.
 7. The method ofclaim 6 wherein the value determined from a combination of the pluralityof measurements comprises an average of the respective values obtainedfrom plurality of measurements.
 8. The method of claim 1, furthercomprising providing a calibration alert if the accuracy of at leastsome of the plurality of measurements is less than the predeterminedaccuracy.
 9. The method of claim 1 wherein the act of making a pluralityof measurements comprises making a plurality of fluid pressuremeasurements.
 10. The method of claim 1 wherein the act of making aplurality of measurements comprises making a plurality of electricalmeasurements.
 11. The method of claim 1 wherein the act of making aplurality of measurements comprises making each of the plurality ofmeasurements in the same measurement environment.
 12. A measurementsystem, comprising: a plurality of measurement devices at least some ofwhich are different from each other, each of the measurement devicesproviding a respective measurement value; and a processor coupled toreceive the measurement values from the plurality of measurementdevices, the processor being operable to derive a measured value from atleast some of the plurality of measurement values, the processor furtherbeing operable to provide information about calibration drift of atleast some of the plurality of calibration devices.
 13. The measurementsystem of claim 12 wherein the plurality of measurement devices haverespective principles of operation that differ from each other.
 14. Themeasurement system of claim 12 wherein the plurality of measurementdevices comprises measurement devices manufactured by differentmanufacturers.
 15. The measurement system of claim 12 wherein theprocessor is operable to compare each of the measurement values from therespective measurement devices to a value derived from a combination ofthe measurement values from a plurality of the respective measurementdevices, and, based on the comparison, to select at least one of themeasurement values to ignore in determining the measured value.
 16. Themeasurement system of claim 12 wherein the value derived from acombination of the measurement values obtained from a plurality of therespective measurement devices comprises an average of the measurementvalues obtained from a plurality of the respective measurement devices.17. The measurement system of claim 12 wherein the processor is operableto provide information about calibration drift of at least some of theplurality of calibration devices by comparing the respective valuesobtained from each of the plurality of measurement devices to a valuederived from a combination of the values obtained from the plurality ofthe respective measurement devices.
 18. The measurement system of claim17 wherein the value derived from a combination of the values obtainedfrom the plurality of the respective measurement devices comprises anaverage of the values obtained from the plurality of the respectivemeasurement devices.
 19. The measurement system of claim 12 wherein theinformation about calibration drift of at least some of the plurality ofcalibration devices provided by the processor comprises a calibrationalert.
 20. The measurement system of claim 12 wherein each of themeasurement devices comprise a respective pressure transducer.
 21. Themeasurement system of claim 12 wherein each of the measurement devicescomprise a respective electrical measurement device.
 22. The measurementsystem of claim 12 wherein the plurality of measurement devices are inthe same measurement environment.
 23. The measurement system of claim 22wherein the each of the measurement devices comprises a respectivepressure transducer, and the measurement system further comprises ahousing to which each of the pressure transducers are fluidly coupled ina symmetrical manner, the housing having a pressure port in fluidcommunication with each of the pressure transducers.
 24. The measurementdevice of claim 23 wherein the pressure port is equidistant from all ofthe pressure transducers.
 25. The measurement device of claim 23 whereinthe pressure transducers are positioned so that the gravity effect onthe overall measurement device remains regardless of the orientation ofthe measurement device.